Radio communication method, local area base station apparatus, mobile terminal apparatus and radio communication system

ABSTRACT

A radio communication method is provided in which a local area base station apparatus, arranged in a wide area where a first carrier frequency is used and forming a local area that is narrower than the wide area, transmits a downlink signal, using a second carrier frequency, which belongs to a different frequency band from the first carrier frequency and which has a wider bandwidth than the first carrier frequency, the local area base station apparatus transmits a detection signal that is used to detect the local area base station apparatus in the mobile terminal apparatus, and the local area base station apparatus transmits the downlink signal for the mobile terminal apparatus with transmission power that is determined based on the path loss of the detection signal received in the mobile terminal apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application and, thereby,claims benefit under 35 U.S.C. § 120 to U.S. patent application Ser. No.14/390,405 filed on Oct. 3, 2014, titled, “RADIO COMMUNICATION METHOD,LOCAL AREA BASE STATION APPARATUS, MOBILE TERMINAL APPARATUS AND RADIOCOMMUNICATION SYSTEM,” which is a national stage application of PCTApplication No. PCT/JP2013/060311, filed on Apr. 4, 2013, which claimspriority to Japanese Patent Application No. 2012-087672 filed on Apr. 6,2012. The contents of the priority applications are incorporated byreference in their entirety.

TECHNICAL FIELD

The present invention relates to a radio communication method, a localarea base station apparatus, a mobile terminal apparatus and a radiocommunication system in a next-generation mobile communication system inwhich local areas are arranged in a wide area.

BACKGROUND ART

In a UMTS (Universal Mobile Telecommunications System) network,long-term evolution (LTE) is under study for the purposes of furtherincreasing high-speed data rates, providing low delay, and so on(non-patent literature 1). In LTE, as multiple access schemes, a schemethat is based on OFDMA (Orthogonal Frequency Division Multiple Access)is used in downlink channels (the downlink), and a scheme that is basedon SC-FDMA (Single Carrier Frequency Division Multiple Access) is usedin uplink channels (the uplink).

Also, successor systems of LTE (referred to as, for example,“LTE-Advanced” or “LTE enhancement” (hereinafter referred to as“LTE-A”)) are under study for the purpose of achieving furtherbroadbandization and increased speed beyond LTE. In LTE-A (Rel-10),carrier aggregation to group a plurality of component carriers (CCs),where the system band of the LTE system is one unit, forbroadbandization, is used. In LTE-A, a HetNet (Heterogeneous Network)configuration to use an interference coordination technique (eICIC:enhanced Inter-Cell Interference Coordination) is under study.

CITATION LIST Non-Patent Literature

Non-Patent Literature 1: 3GPP 9 TR 25.913 “Requirements for Evolved UTRAand Evolved UTRAN”

SUMMARY OF THE INVENTION Technical Problem

Now, in cellular systems such as W-CDMA, LTE (Rel. 8), and successorsystems of LTE (for example, Rel. 9 and Rel. 10), the radiocommunication schemes (radio interfaces) are designed to support wideareas. In the future, it is expected that high-speed wireless servicesby means of near-field communication will be provided in local areassuch as indoors, shopping malls and so on, in addition to cellularenvironment such as given above. Consequently, there is a demand todesign new radio communication schemes that are specifically designedfor local areas, so that capacity can be secured with local areas whilecoverage is secured with a wide area.

The present invention has been made in view of the above, and it istherefore an object of the present invention to provide a radiocommunication method, a local area base station apparatus, a mobileterminal apparatus and a radio communication system that can providehighly efficient local area radio access.

Solution to Problem

The radio communication method according to the present invention is aradio communication method in which a local area base station apparatus,arranged in a wide area where a first carrier frequency is used andforming a local area that is narrower than the wide area, transmits adownlink signal, using a second carrier frequency, which belongs to adifferent frequency band from the first carrier frequency and which hasa wider bandwidth than the first carrier frequency, and this radiocommunication method includes the steps in which: the local area basestation apparatus transmits a detection signal that is used to detectthe local area base station apparatus in the mobile terminal apparatus;and the local area base station apparatus transmits the downlink signalfor the mobile terminal apparatus with transmission power that isdetermined based on a path loss of the detection signal received in themobile terminal apparatus.

The local area base station apparatus according to the present inventionis a local area base station apparatus that is arranged in a wide areawhere a first carrier frequency is used and that forms a local area,which is narrower than the wide area, the local area base stationapparatus and this local area base station apparatus has: a downlinksignal transmission section that transmits a downlink signal, using asecond carrier frequency, which belongs to a different frequency bandfrom the first carrier frequency and which has a wider bandwidth thanthe first carrier frequency; and a detection signal transmission sectionthat transmits a detection signal that is used to detect the local areabase station apparatus in the mobile terminal apparatus, and thedownlink signal transmission section transmits the downlink signal forthe mobile terminal apparatus with transmission power determined basedon a path loss of the detection signal received in the mobile terminalapparatus.

The mobile terminal apparatus according to the present invention is amobile terminal apparatus that receives, from a local area base stationapparatus arranged in a wide area where a first carrier frequency isused and forming a local area that is narrower than the wide area, adownlink signal, using a second carrier frequency, which belongs to adifferent frequency band from the first carrier frequency and which hasa wider bandwidth than the first carrier frequency, and this mobileterminal apparatus has: a detection signal receiving section thatreceives, from the local area base station apparatus, a detection signalthat is used to detect the local area base station apparatus in themobile terminal apparatus; and a downlink signal receiving section thatreceives the downlink signal transmitted from the local area basestation apparatus with transmission power determined based on a pathloss of the detection signal.

The radio communication system according to the present invention is aradio communication system in which a local area base station apparatus,arranged in a wide area where a first carrier frequency is used andforming a local area that is narrower than the wide area, transmits adownlink signal, using a second carrier frequency, which belongs to adifferent frequency band from the first carrier frequency and which hasa wider bandwidth than the first carrier frequency, and, in this radiocommunication system: the local area base station apparatus transmits adetection signal that is used to detect the local area base stationapparatus in the mobile terminal apparatus; and the local area basestation apparatus transmits the downlink signal for the mobile terminalapparatus with transmission power that is determined based on a pathloss of the detection signal received in the mobile terminal apparatus.

Technical Advantage of the Invention

According to the present invention, it is possible to provide a radiocommunication method, a local area base station apparatus, a mobileterminal apparatus and a radio communication system that can providehighly efficient local area radio access. In particular, by executingtransmission power control on the downlink of a local area, it ispossible to make the uplink and downlink coverages in the local areanearly symmetrical.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram to explain a system band in an LTE-A system;

FIG. 2 is a diagram to show a configuration to arrange many small cellsin a macro cell;

FIG. 3A provides a diagram to show a heterogeneous networkconfiguration;

FIG. 3B provides a diagram to show another heterogeneous networkconfiguration;

FIG. 4 is a diagram to show carriers used in a wide area and a localarea;

FIG. 5 shows a table to list differences between a wide area and a localarea;

FIG. 6 is a diagram to show an arrangement configuration of thediscovery signals and DACH;

FIG. 7 is a diagram to explain the uplink and downlink coverages of alocal area;

FIG. 8 is a conceptual diagram of transmission power control in thedownlink of a local area;

FIG. 9 is a sequence diagram to show a radio communication method in thedownlink of a local area according to a first example;

FIG. 10 is a sequence diagram to show a radio communication method inthe downlink of a local area according to a second example;

FIG. 11 is a diagram to show difference in transmission power betweenthe PDSCH/EPDCCH and the CSI-RS;

FIG. 12A provides a diagram to show a first example of a CSI-RSarrangement configuration;

FIG. 12B provides a diagram to show a second example of a CSI-RSarrangement configuration;

FIG. 12C provides a diagram to show a third example of a CSI-RSarrangement configuration;

FIG. 13 provides diagrams to show examples of CSI-RS arrangementconfiguration;

FIG. 14 is a diagram to explain an example of a system configuration ofa radio communication system;

FIG. 15 is a diagram to show a configuration of a mobile terminalapparatus;

FIG. 16 is a diagram to show a configuration of a wide area base stationapparatus; and

FIG. 17 is a diagram show a configuration of a local area base stationapparatus.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is a diagram to show a layered bandwidth configuration defined inLTE-A. The example shown in FIG. 1 is a layered bandwidth configurationthat is assumed when an LTE-A system having a first system band formedwith a plurality of fundamental frequency blocks (hereinafter referredto as “component carriers”), and an LTE system having a second systemband formed with one component carrier, coexist. In the LTE-A system,for example, radio communication is performed in a variable systembandwidth of 100 MHz or below, and, in the LTE system, for example,radio communication is performed in a variable system bandwidth of 20MHz or below. The system band of the LTE-A system includes at least onecomponent carrier, where the system band of the LTE system is one unit.Widening the band by way of gathering a plurality of component carriersin this way is referred to as “carrier aggregation.”

For example, in FIG. 1, the system band of the LTE-A system is a systemband to include bands of five component carriers (20 MHz×5=100 MHz),where the system band (base band: 20 MHz) of the LTE system is onecomponent carrier. In FIG. 1, mobile terminal apparatus UE (UserEquipment) #1 is a mobile terminal apparatus to support the LTE-A system(and also support the LTE system), and is able to support a system bandup to 100 MHz. UE #2 is a mobile terminal apparatus to support the LTE-Asystem (and also support the LTE system), and is able to support asystem band up to 40 MHz (20 MHz×2=40 MHz). UE #3 is a mobile terminalapparatus to support the LTE system (and not support the LTE-A system),and is able to support a system band up to 20 MHz (base band).

Now, future systems may anticipate a configuration to arrange numeroussmall cell S's in a macro cell, just as shown in FIG. 2. In this case,the small cell S's need to be designed taking into account capacityversus network costs. The network costs may include, for example, thecost of installing the network nodes, backhaul link and so on, theoperation cost for cell planning and maintenance support, the powerconsumption on the network side, and so on. Also, as demands apart fromcapacity, small cell S's are required to support saved power consumptionon the mobile terminal apparatus side, random cell planning, and so on.

When small cells are arranged in a macro cell M, two kinds ofheterogeneous network (hereinafter referred to as “HetNet”)configurations may be possible, as shown in FIGS. 3A and 3B. In thefirst HetNet configuration shown in FIG. 3A, the small cell S's arearranged such that the macro cell M and the small cell S's use the samecarrier frequency. In the second HetNet configuration shown in FIG. 3B,the small cell S's are arranged such that the macro cell M and the smallcell S's use different carrier frequencies. In the second HetNetconfiguration, the small cell S's use a dedicated carrier frequency, sothat it is possible to secure coverage with the macro cell M and securecapacity with the small cell S's. It is expected that, in the future(Rel. 12 and later versions), this second HetNet configuration willbecome more important.

An example of carriers used in the second HetNet configuration will bedescribed with reference to FIG. 4. In the following, the macro cell Mand small cell S's in FIG. 3B will be referred to as “the wide area” and“the local areas,” respectively. The wide area may be a sector and soon, besides a macro cell, and the local areas may be pico cells, nanocells, femto cells, micro cells and so on, besides small cells. Theradio base stations to cover (referred to as “coverage range”) the widearea and the local areas will be referred to as the wide area basestation apparatus and the local area base station apparatuses,respectively.

As shown in FIG. 4, the carrier that is used in the wide area in thesecond HetNet configuration (hereinafter referred to as “the wide areacarrier”) is an existing carrier wave having a relatively narrowbandwidth (for example, 2 MHz) in a predetermined frequency band. Thewide area carrier is transmitted with relatively high transmission powerso that a large and wide area can be covered. This wide area carrier isalso referred to as a legacy carrier, a coverage carrier and so on.

The carrier that is used in the local areas in the second HetNetconfiguration (hereinafter referred to as “the local area carrier”) is acarrier wave having a relatively wide bandwidth (for example, 3.5 GHz)in a different frequency band from that of the wide area carrier (inFIG. 4, in a frequency band higher than that of the wide area carrier).The local area carrier has a wide bandwidth for improved capacity, andtherefore is transmitted with relatively low transmission power. Thislocal area carrier is also referred to as an additional carrier, anextension carrier, a capacity carrier and so on.

In this second HetNet configuration, as shown in FIG. 5, it is likelythat the wide area and the local areas have different requirements andso on. For example, the wide area has a limited bandwidth, so thatspectral efficiency is very important. By contrast with this, with thelocal areas, it is easy to take a wide bandwidth, so that, if a widebandwidth can be secured, the significance of spectral efficiency is notas high as in the wide area. While the wide area needs to support highmobility such as represented by cars and/or the like, the local areashave only to support low mobility. The wide area needs to secure widecoverage. Although it is preferable to secure wide coverage with thelocal areas as well, the wide area can cover up the shortage ofcoverage.

In the wide area, significant capacity differences exist between thewide area base station apparatus and a mobile terminal apparatus, sothat the difference in the maximum transmission power between the uplinkand the downlink grows, and the uplink and the downlink haveasymmetrical transmission power. In the local areas, there are onlyinsignificant capacity differences between the local area base stationapparatuses and a mobile terminal apparatus, so that the difference ofthe maximum transmission power between the uplink and the downlinkbecomes smaller, and the uplink and the downlink have nearly symmetricaltransmission power. Furthermore, in the wide area, the number ofconnecting users per cell is high and cell planning is applied, so thatthere is little variation of traffic. By contrast with this, in thelocal areas, the number of connecting users per cell is low, andfurthermore there is a possibility that cell planning is not applied, sothat there is significant variation of traffic. In this way, sinceoptimal requirements for the local areas are different from those of thewide area, it is necessary to design radio communication schemes thatare specifically designed for the local areas.

Considering interference that arises from saved power consumption andrandom cell planning, it is preferable to configure the radiocommunication scheme for the local areas to assume non-transmission whenthere is no traffic. Consequently, as shown in FIG. 6, the radiocommunication scheme for the local areas is expected to be designed asUE-specific as possible. To be more specific, the radio communicationscheme for the local areas is designed based on the EPDCCH (EnhancedPhysical Downlink Control Channel), the PDSCH (Physical Downlink SharedChannel), and the DM-RS (DeModulation-Reference Signal), without usingcell-specific signals such as the PSS/SSS (Primary SynchronizationSignal/Secondary Synchronization Signal), the CRS (Cell-specificReference Signal), the PDCCH (Physical Downlink Control Channel) in LTE.

Here, the EPDCCH (enhanced downlink control signal) refers to a downlinkcontrol signal that is frequency-division-multiplexed with the PDSCH(downlink data signal). Like the PDSCH, the EPDCCH is demodulated usingthe DM-RS, which is a user-specific demodulation reference signal. Notethat the EPDCCH may be referred to as an FDM-type PDCCH or may bereferred to as a UE-PDCCH. In FIG. 6, the PDSCH, the EPDCCH, the DM-RSand so on are shown as the UE-specific L1/L2 signals.

Also, in the radio communication scheme for the local areas, as shown inFIG. 6, in addition to defining discovery signals on the downlink,defining the DACH (Direct Access Channel) on the uplink is also understudy. Here, the discovery signals refers to detection signals that areused to allow a mobile terminal apparatus to detect the local area basestation apparatuses. Also, the DACH refers to a dedicated access channelfor the local area base station apparatuses. By means of the DACH, thereceived power of discovery signals at a mobile terminal apparatus andso on are transmitted.

As shown in FIG. 6, downlink discovery signals are transmitted in arelatively long cycle (for example, on the order of several seconds), sothat a mobile terminal apparatus is able to reduce the number of timesof measurement and save battery. As for the uplink DACH, radio resourcesare allocated in a shorter cycle than discovery signals. By means ofthis, uplink connection is established quickly when traffic is producedin the mobile terminal apparatus.

The signal arrangement shown in FIG. 6 is simply an example and is by nomeans limiting. For example, it is equally possible to allocate radioresources to the DACH in the same cycle as discovery signals (forexample, in a cycle of several seconds). Also, a discovery signal may bereferred to as the PDCH (Physical Discovery Channel), the BS (BeaconSignal), the DPS (Discovery Pilot Signal) and so on. The DACH is notlimited to a particular name.

In the local areas of the second HetNet configuration such as onesdescribed above, cases might occur where the uplink and downlinkcoverages becomes asymmetrical. As has been described with reference toFIG. 5, in a local area, differences between the local area base stationapparatus and a mobile terminal apparatus in terms of capacity areinsignificant, so that the difference of the maximum transmission powerbetween the uplink and the downlink becomes small, and, generally, theuplink and the downlink have nearly symmetrical transmission power.However, as shown in FIG. 7, in the uplink of a local are, it ispossible to execute transmission power control to narrow the bandwidthof the local area carrier shown in FIG. 4 and increase the transmissionpower. As a result of this, as shown in FIG. 7, there is a problem thatthe uplink transmission power becomes obviously bigger than the downlinktransmission power and makes the uplink and downlink coverages in thelocal area asymmetrical.

So, the present inventors have arrived at the present invention toprevent the uplink and downlink coverages from being asymmetrical in alocal area, where the transmission power difference between the localarea base station apparatus and a mobile terminal apparatus isinsignificant, and where the uplink and the downlink generally havenearly symmetrical transmission power. That is, a gist of the presentinvention is to execute, even in the downlink of a local area,transmission power control to narrow the bandwidth of the local areacarrier shown in FIG. 4 and increase the transmission power, so that theuplink and downlink coverages in the local area become nearlysymmetrical.

Now, the transmission power control scheme in the downlink of a localarea according to the present embodiment will be described. Note thatthe following description will assume a radio communication system wherea plurality of local areas are arranged in a wide area (see FIG. 14).Assume that this radio communication system adopts the above-describedsecond HetNet configuration, and that, in the local areas, a local areacarrier (second carrier frequency) that has a wider bandwidth than thewide area carrier in a different frequency band from that of the widearea carrier (first carrier frequency) is used.

FIG. 8 is a conceptual diagram of the transmission power control in thedownlink of the local areas according to the present embodiment. Asshown in FIG. 8, in the downlink of a local area, the discovery signalis transmitted with certain transmission power so as to maximize thecoverage. By means of this, more mobile terminal apparatuses 10 are ableto detect the discovery signal.

Meanwhile, in FIG. 8, the transmission power of downlink signals such asthe EPDCCH (enhanced downlink control signal) and the PDSCH (downlinkdata signal) is controlled adaptively. For example, as for downlinksignals for a mobile terminal apparatus 10 located near a local areabase station apparatus 30, a local area carrier (see FIG. 4) having awide bandwidth and low transmission power may be used as is, to securehigh capacity while maintaining small coverage. As for downlink signalsfor a mobile terminal apparatus 10 located at a distance from a localarea base station apparatus 30, the transmission power of the local areacarrier (see FIG. 4) may be increased by narrowing the bandwidth, andthe capacity may be reduced by expanding the coverage.

Next, the radio communication method in the downlink of the local areasaccording to the present embodiment will be described with reference toFIG. 9 and FIG. 10.

FIG. 9 is a sequence diagram to show the radio communication method inthe downlink of the local areas according to a first example. Referringto FIG. 9, although the wide area base station apparatus 20 and a localarea base station apparatus 30 are connected via a wire interface suchas an X2 interface, they may be connected via a radio interface as well.Also, assume that a mobile terminal apparatus 10 is connected with thewide area base station apparatus 20 and a local area base stationapparatus 30 via a radio interface.

As shown in FIG. 9, a local area base station apparatus 30 receivescontrol information for discovery signal (DS) transmission from the widearea base station apparatus 20 (step S101). The control information fordiscovery signal transmission includes, for example, radio resources anda signal sequence for transmitting the discovery signal. Note that everylocal area is provided with a signal sequence of a discovery signal, andthe local area is identified with this signal sequence.

The mobile terminal apparatus 10 receives local area controlinformation, including, for example, control information for discoverysignal (DS) reception, control information for DACH transmission, andcontrol information for EPDCCH reception, from the wide area basestation apparatus 20 (step S102).

Here, the control information for discovery signal reception includesradio resources, signal sequence and so on for receiving the discoverysignal from the local area base station apparatus 30. The controlinformation for discovery signal reception may include the transmissionpower of the discovery signal from the local area base station apparatus30. Also, the control information for DACH transmission includes theradio resources, the DM-RS sequence and so on allocated to the DACH. Thecontrol information for EPDCCH reception includes the radio resources,the DM-RS sequence and so on for receiving downlink control information(DCI) using an EPDCCH from the local area base station apparatus 30.

The mobile terminal apparatus 10 receives the discovery signal from thelocal area base station apparatus 30 based on the control informationfor discovery signal reception received in step S102, and measures thereceived power of these discovery signal (step S103). As describedabove, the discovery signal is transmitted in a predetermined cycle fromthe local area base station apparatus 30 (for example, in a cycle ofseveral seconds). Note that, for the received power of the discoverysignal, for example, the SINR (Signal-to-Interference and Noise powerRatio) of the discovery signal is used.

The mobile terminal apparatus 10 calculates the path loss of thediscovery signal based on the received power of the discovery signal(step S104). To be more specific, the mobile terminal apparatus 10calculates the difference between the transmission power value of thediscovery signal received from the wide area base station apparatus 20in step S102, and the received power value of the discovery signalmeasured in step S103, as the path loss of the discovery signal.

The mobile terminal apparatus 10 transmits the received power or pathloss of the discovery signal to the local area base station apparatus30, using the DACH, based on the control information for DACHtransmission received in step S102 (step S105). Here, the initialtransmission power of the DACH may be determined based on the path losscalculated in step S104. For example, the initial transmission power ofthe DACH may be the value adding a predetermined offset to the abovepath loss, equal to or lower than the maximum transmission power of themobile terminal apparatus 10.

The local area base station apparatus 30 determines the initialtransmission power of downlink signals for the mobile terminal apparatus10 based on the path loss of the discovery signal at the mobile terminalapparatus 10 (step S106). For example, the initial transmission power ofdownlink signals may be the value adding a predetermined offset to theabove path loss, equal to or lower than the maximum transmission powerof the local area base station apparatus 30. Note that the downlinksignals, the initial transmission power of which is to be determined,may be the EPDCCH (enhanced downlink control signal) and the PDSCH(downlink data signal) that are frequency-division-multiplexed.

Note that the path loss of the discovery signal may be calculated in themobile terminal apparatus 10 and transmitted to the local area basestation apparatus 30, or may be calculated in the local area basestation apparatus 30. The local area base station apparatus 30 receivethe received power value of the discovery signal from the mobileterminal apparatus 10, calculates the path loss of the discovery signalfrom the received power value as received and the transmission powervalue of the discovery signal, and determined the initial transmissionpower of downlink signals from the path loss calculated.

The local area base station apparatus 30 transmits the downlink signalswith the initial transmission power determined in step S106 (step S107).To be more specific, the local area base station apparatus 30 transmitsthe EPDCCH and the PDSCH with the initial transmission power representedby initial transmission power information from the wide area basestation apparatus 20. Based on the DCI (Downlink Control Information)transmitted using the EPDCCH, the mobile terminal apparatus 10identifies and receives the PDSCH allocated to the mobile terminalapparatus 10.

Note that a CSI-RS (Channel State Information Reference Signal) ismultiplexed over the downlink signals transmitted from the local areabase station apparatus 30. The CSI-RS refers to a measurement referencesignal for measuring the channel state between the local area basestation apparatus 30 and the mobile terminal apparatus 10.

The mobile terminal apparatus 10 measures the received power of theCSI-RS multiplexed upon the downlink signals, and generates CSI (ChannelState Information) based on the measured received power (step S108). Thereceived power of the CSI-RS may be, for example, the SINR(Signal-to-Interference and Noise power Ratio) of the CSI-RS.

The CSI refers to channel state information that represents the channelstate between the mobile terminal apparatus 10 and the local area basestation apparatus 30, such as a CQI (Channel Quality Indicator), a PMI(Precoding Matrix Indicator), an RI (Rank Indicator) and so on. The CQIrefers to a value that is calculated based on the SINR of the CSI-RSfrom the local area base station apparatus 30, and every value isassociated with a modulation and coding scheme (MCS).

The mobile terminal apparatus 10 transmits the CSI generated in stepS108 to the local area base station apparatus 30 (step S109). Note thatthis transmission of CSI may be carried out using a PUCCH (PhysicalUplink Control Channel, which is an uplink control signal) or may becarried out using a PUSCH (Physical Uplink Shared Channel, which is anuplink data signal).

The local area base station apparatuses 30 determine the transmissionpower of downlink signals based on the CSI received from the mobileterminal apparatus 10 in step S109 and the above-described path loss ofthe discovery signal (step S110). The downlink signals, the transmissionpower of which is to be determined, may be the EPDCCH and the PDSCH thatare frequency-division-multiplexed. Note that the local area basestation apparatus 30 may determine the offset value for the currentdownlink signal transmission power based on the above CSI and path loss.

Also, the local area base station apparatus 30 may determine themodulation coding scheme (MCS) to apply to the downlink signals based onthe CQI received from the mobile terminal apparatus 10 in step S109.

The local area base station apparatus 30 transmits the downlink signalswith the transmission power determined in step S110 (step S111). Thelocal area base station apparatus 30 may also transmit the downlinksignals by the MCS determined based on the CQI.

As described above, with the radio communication method in the downlinkof a local area according to the first example shown in FIG. 9, even inthe downlink of a local area, the transmission power of downlink signalsis controlled adaptively based on the path loss of the discovery signal.In this way, by controlling the transmission power of a local areacarrier not only in the uplink of the local area, but also in thedownlink the local area as well, it becomes possible to make the uplinkand downlink coverages in the local area nearly symmetrical.

Also, with the radio communication method in the downlink of a localarea according to the first example shown in FIG. 9, the local area basestation apparatus 30, which the mobile terminal apparatus 10 connectswith, determines the transmission power of downlink signals.Consequently, compared to the case of determining transmission power inthe wide area base station apparatus 20, it is possible to control thetransmission power of downlink signals more quickly.

FIG. 10 is a sequence diagram to show the radio communication method inthe downlink of a local area according to a second example. The radiocommunication method shown in FIG. 10 is different from the radiocommunication method shown in FIG. 9 in that the initial transmissionpower and transmission power of downlink signals is determined in thewide area base station apparatus 20, not in the local area base stationapparatus 30. Now, differences from FIG. 9 will be described mainly.

Note that, in FIG. 10, connection (for example, higher layer connectionsuch as RRC connection) is established between the mobile terminalapparatus 10 and the wide area base station apparatus 20. Also, stepsS201 to S204 of FIG. 10 are the same as steps S101 to S104 of FIG. 9.

As shown in FIG. 10, the mobile terminal apparatus 10 transmits thereceived power or path loss of the discovery signal to the wide areabase station apparatus 20 using higher layer signaling such as RRCsignaling (step S205).

The wide area base station apparatus 20 determines the initialtransmission power of downlink signals from the local area base stationapparatus 30 to the mobile terminal apparatus 10 based on the path lossof the discovery signal in the mobile terminal apparatus 10 (step S206).For example, the initial transmission power of downlink signals from thelocal area base station apparatus 30 may be a value adding apredetermined offset to the above path loss, equal to or lower than themaximum transmission power of the local area base station apparatus 30.Note that the downlink signals, the initial transmission power of whichis to be determined, may be the EPDCCH and the PDSCH that arefrequency-division-multiplexed.

Note that the path loss of the discovery signal may be calculated in themobile terminal apparatus 10 and transmitted to the wide area basestation apparatus 20, or may be calculated in the wide area base stationapparatus 20. The wide area base station apparatus 20 receive thereceived power value of the discovery signal from the mobile terminalapparatus 10, calculates the path loss of the discovery signal from thereceived power value as received and the transmission power value of thediscovery signal, and determined the initial transmission power ofdownlink signals from the path loss calculated.

The wide area base station apparatus 20 transmits transmission powerinformation to represent the determined initial transmission power tothe local area base station apparatus 30 via a wire interface such as anX2 interface (step S207).

The local area base station apparatus 30 transmits downlink signals withthe initial transmission power represented by the transmission powerinformation from the wide area base station apparatus 20 (step S208). Tobe more specific, the local area base station apparatus 30 transmits atleast one of the EPDCCH and the PDSCH with the initial transmissionpower represented by the transmission power information from the widearea base station apparatus 20.

The mobile terminal apparatus 10 measures the received power of theCSI-RS multiplexed over the downlink signals, and, based on the receivedpower measured, generates CSI including a CQI and so on (step S209).

The mobile terminal apparatus 10 transmits the CSI generated in stepS209 to the wide area base station apparatus 20 (step S210). Note thatthis transmission of CSI may be carried out using higher layer signalingsuch as RRC signaling.

The wide area base station apparatus 20 determines the transmissionpower of downlink signals from the local area base station apparatus 30to the mobile terminal apparatus 10 based on the CSI received from themobile terminal apparatus 10 in step S210 and the path loss of thediscovery signal described above (step S211). Note that the downlinksignals from the local area base station apparatus 30, the transmissionpower of which is to be determined, may be the EPDCCH and the PDSCH thatare frequency-division-multiplexed. Note that the local area basestation apparatus 30 may determine the offset value for the currentdownlink signal transmission power from the local area base stationapparatus 30 based on the above CSI and path loss.

Also, the wide area base station apparatus 20 may determine themodulation coding scheme (MCS) to apply to the downlink signals from thelocal area base station apparatus 30 based on the CQI received from themobile terminal apparatus 10 in step S210.

The wide area base station apparatus 20 transmits transmission powerinformation to represent the determined transmission power to the localarea base station apparatus 30 via a wire interface such as an X2interface (step S212). Also, the wide area base station apparatus 20 maytransmit MCS information to represent the determined MCS to the localarea base station apparatus 30.

The local area base station apparatus 30 transmits downlink signals withthe transmission power represented by the transmission power informationfrom the wide area base station apparatus 20 (step S213). Also, thelocal area base station apparatus 30 may transmit downlink signalsincluding the EPDCCH and the PDSCH by the MCS represented by the MCSinformation from the wide area base station apparatus 20.

With the radio communication method in the downlink of a local areaaccording to the second example shown in FIG. 10, the wide area basestation apparatus 20, which forms the wide area where the local areabase station apparatus 30 is arranged, determines the transmission powerof downlink signals. By this means, it is possible to determine moreoptimal downlink signal transmission power taking into account the loadbalance between the local areas and so on. Also, by allowing the widearea base station apparatus 20 to determine the transmission power ofdownlink signals for the local area base station apparatus 30 under thewide area base station apparatus 20, downlink signal transmission powercontrol in CoMP (Coordinated Multiple Point) is made possible. Notethat, the radio communication methods of FIG. 9 and FIG. 10 may becombined, by, for example, combining step S108 and earlier steps in FIG.9 and step S210 and later steps in FIG. 10, or step S209 and earliersteps in FIG. 10 and step S109 and later steps in FIG. 9, and so on.

Next, a CSI reporting scheme that is suitable with the transmissionpower control scheme in the downlink of a local area according to thepresent embodiment will be described with reference to FIG. 11.

FIG. 11 is a diagram to show downlink signal transmission power that issubject to adaptive control in a local area according to the presentembodiment. As shown in FIG. 11, in the local area according to thepresent embodiment, the transmission power of the EPDCCH or the PDSCHincreases adaptively, while the transmission power of the CSI-RS is keptconstant. The CSI-RS is used in channel estimation in the mobileterminal apparatus, so that it is preferable to maintain thetransmission power of the CSI-RS constant so that channel statevariations can be detected.

In the case shown in FIG. 11, the transmission power difference betweenthe CSI-RS having transmission power that is kept low, and the EPDCCH orthe PDSCH having transmission power that increases adaptively,increases. By this means, the received power difference between theCSI-RS and the EPDCCH or the PDSCH at the mobile terminal apparatus 10increases.

For example, although the SINR of the CSI-RS becomes poorer at −5 dB andbelow, the adaptive control of the transmission power of the EPDCCH orthe PDSCH may make the SINR of the EPDCCH or the PDSCH good at +5 dB andabove, and the received power difference between the CSI-RS and theEPDCCH or the PDSCH may increase. In this case, the CQI that is measuredusing the CSI-RS becomes dissimilar to the actual received quality ofthe EPDCCH or the PDSCH, and therefore the accuracy of link adaptation(MCS selection) based on the CQI may decrease. So, it may be possible toemploy CSI reporting schemes such as ones described below.

With a first CSI reporting scheme, the difference (ΔS) between theadaptively-controlled transmission power of the EPDCCH or the PDSCH, andthe transmission power of the CSI-RS allocated to the same resourceblocks with this EPDCCH or PDSCH is reported in advance from the widearea base station apparatus 20 or the local area base station apparatus30 to the mobile terminal apparatus 10.

For example, the transmission power difference ΔS may be reported fromthe wide area base station apparatus 20 in step S102 of FIG. 9 or instep S202 of FIG. 10. Alternatively, the transmission power differenceΔS may be reported from the local area base station apparatus 30 in theaccess step triggered by the DACH. Based on the transmission powerdifference ΔS reported, the mobile terminal apparatus 10 corrects theCQI value calculated using the SINR of the CSI-RS and reports CSIincluding the corrected CQI. For example, the mobile terminal apparatus10 corrects the CQI value to add ΔS to the SINR of the CSI-RS.

With the first CSI reporting scheme, it is possible to prevent a CQI oflow accuracy from being reported and damaging the accuracy of linkadaptation (MCS selection), without changing the conventional CQIconfiguration.

With a second CSI reporting scheme, the measurement range of CQIs(channel quality indicators) using the received power of the CSI-RS isexpanded. For example, it may be possible to expand the measurementrange of CQIs so as to be able to cope with cases up to where the SINRof the CSI-RS is −20 dB. A conventional CQI is identified with a CQIindex, which is given in 16 ranks from 0 to 15, and four bits aresecured for the CQI index. With the second CSI reporting scheme, thenumber of CQI index bits is further expanded, and the CQI index value tocorrespond to the expanded measurement range is reported. By this means,even when the CQI that is measured using the CSI-RS becomes somewhatdissimilar to the actual received quality of the EPDCCH or the PDSCH, itis possible to keep the CQI within the CQI measurement range, so thatthe accuracy of link adaptation (MCS selection) based on the CQIimproves.

With the second CSI reporting scheme, without reporting the transmissionpower difference ΔS between the EPDCCH or the PDSCH and the CSI-RS, itis possible to report a CQI of high accuracy and prevent the accuracy oflink adaptation from decreasing.

Next, a CSI-RS arrangement configuration that is suitable for thetransmission power control scheme in the downlink of a local areaaccording to the present embodiment will be described with reference toFIG. 12 and FIG. 13.

FIG. 12 is a diagram to show an example of a CSI-RS arrangementconfiguration in a local area according to the present embodiment. Inthe local area, the CSI-RS is transmitted using a local area carrierhaving a wide bandwidth and low transmission power (see FIG. 4). Asdescribed above, unlike the EPDCCH or the PDSCH, the CSI-RS is notsubject to adaptive transmission power control. Consequently, when theCSI-RS is transmitted in using a local area carrier, the mobile terminalapparatus 10 is unable to receive the CSI-RS with sufficient receivedpower, and therefore the accuracy of CSI-RS measurement deteriorates.So, as shown in FIG. 12A, it may be possible to insert the CSI-RS inincreased density.

FIG. 12A shows an example of a CSI-RS arrangement configuration in asubframe where the density of insertion is increased. As shown in FIG.12A, the CSI-RS to be transmitted in a local area is arranged at shortersubcarrier intervals than the subcarrier intervals at which the CSI-RSto be transmitted in the wide area is arranged, in specific OFDM symbolsin a subframe.

For example, in FIG. 12A, the CSI-RS is arranged every three subcarriersin the ninth and tenth OFDM symbols from the top of the subframe. In thewide area, in the event of two antenna ports, the CSI-RS is arrangedevery 12 subcarriers. Consequently, with the arrangement shown in FIG.12A, the density of the CSI-RS to be inserted increases more in thelocal area than in the wide area.

As shown in FIG. 12A, by making the subcarrier interval to arrange theCSI-RS smaller and increasing the density of the CSI-RS to insert, it ispossible to prevent the accuracy of CSI-RS measurement in the mobileterminal apparatus 10 from decreasing even when the CSI-RS istransmitted using a local area carrier having a wide bandwidth andhaving transmission power that becomes lower.

Also, as shown in FIG. 11, it may be possible that, when the EPDCCH orthe PDSCH with transmission power that increases adaptively and theCSI-RS are arranged in neighboring subcarriers, the power boost of theEPDCCH or the PDSCH is prevented. So, it may be possible to arrange theCSI-RS as shown in FIG. 12B and FIG. 12C.

In FIG. 12B, an example of arrangement configuration where specific OFDMsymbols in a subframe are occupied with the CSI-RS. As shown in FIG.12B, the CSI-RS to be transmitted in a local area are arranged in allsubcarriers constituting the resource block width in specific OFDMsymbol in a subframe.

In the arrangement configuration shown in FIG. 12B, in specific OFDMsymbols in a subframe, the EPDCCH or the PDSCH with transmission powerthat increases adaptively and the CSI-RS with constantly lowtransmission power are never arranged in neighboring subcarrier.Consequently, it is possible to prevent the CSI-RS from blocking thepower boost of the EPDCCH or the PDSCH. Also, since the CSI-RS isarranged in all the subcarriers constituting the resource block width,so that, even when the CSI-RS is transmitted using a local area carrierof a wide bandwidth, it is still possible to prevent the decrease of theaccuracy of CSI-RS measurement in the mobile terminal apparatus 10.

In FIG. 12C shows an example of arrangement configuration where, inspecific OFDM symbols in a subframe, the CSI-RS and the EPDCCH or thePDSCH are prevented from being frequency-division-multiplexed. As shownin FIG. 12C, the CSI-RS that is transmitted in a local area are arrangedin part of the subcarriers constituting the resource block width, inspecific OFDM symbols in a subframe. At this time, muting resources(zero-power CSI-RS) are arranged in the rest of the subcarriers so thatthe PDSCH is not arranged in the rest of the subcarriers.

In the arrangement configuration shown in FIG. 12C, in specific OFDMsymbols in a subframe, muting resources are arranged in subcarrierswhere the CSI-RS is not arranged. Consequently, in specific OFDM symbolsin a subframe, the EPDCCH or the PDSCH with transmission power thatincreases adaptively and the CSI-RS with constantly low transmissionpower are never arranged in neighboring subcarrier. As a result, it ispossible to prevent the CSI-RS from blocking the power boost of theEPDCCH or the PDSCH.

Note that the CSI-RS arrangements shown in FIG. 12A to FIG. 12C aresimply examples, and are by no means limiting. For example, in FIG. 12Ato FIG. 12C, the CSI-RS may be arranged in OFDM symbols apart from theninth and tenth OFDM symbols in the subframe. Also, in FIG. 12A, if thesubcarrier interval to arrange the CSI-RS is shorter than in the widearea, the CSI-RS may be arranged in any subcarriers. Also, in FIG. 12C,in specific OFDM symbols in a subframe, if muting resources are arrangedin subcarriers where the CSI-RS is not arranged, the CSI-RS may bearranged in any subcarriers.

Also, as shown in FIG. 12A to FIG. 12C, a local area is studied not toarrange the PDCCH in maximum three OFDM symbols from the top of asubframe. Consequently, the CSI-RS may be arranged in this resourceregion. Also, the arrangement configuration for the DM-RS, which is aUE-specific demodulation reference signal, is not limited to thearrangements shown in FIG. 12A to FIG. 12C either.

FIG. 13 is a diagram to show an example of a CSI-RS arrangementconfiguration where carrier aggregation is executed between a wide areaand a local area. When carrier aggregation is executed between the widearea base station apparatus 20 and a local area base station apparatus30, it may occur that the wide area carrier and the local area carrierdescribed above with reference to FIG. 4 belong to different componentcarriers.

In this case, as shown in FIG. 13, the CSI-RS that is transmitted in thelocal area may be arranged in a subframe apart from the subframe wherethe CSI-RS to be transmitted in the wide area is arranged. For example,in FIG. 13, the CSI-RS of the wide area is arranged in subframes 1 and3, while the CSI-RS of the local area is arranged in subframe 2.

When carrier aggregation is executed, the component carrier (CC 1) ofthe wide area and the component carrier (CC 2) of the local area havedifferent frequency bands. Consequently, by arranging CSI-RSs indifferent subframes between component carriers, it becomes possible torealize frequency hopping virtually.

Note that the CSI-RS arrangements shown in FIG. 13 are simply examples,and are by no means limiting. For example, in FIG. 13, the CSI-RS may bearranged in OFDM symbols apart from the ninth and tenth OFDM symbols ina subframe. Also, given a specific OFDM symbol, the CSI-RS may bearranged in any subcarriers.

Now, the radio communication system according to the present embodimentwill be described in detail. FIG. 14 is a diagram to explain a systemconfiguration of the radio communication system according to the presentembodiment. Note that the radio communication system shown in FIG. 14 isa system to accommodate, for example, the LTE system or SUPER 3G. Inthis radio communication system, carrier aggregation to group aplurality of fundamental frequency blocks as one, where the system bandof the LTE system is one unit, is used. Also, this radio communicationsystem may be referred to as “IMT-Advanced,” or may be referred to as“4G,” “FRA” (Future Radio Access) and so on.

As shown in FIG. 14, the radio communication system 1 has a wide areabase station apparatus 20, which forms a wide area C1, and a pluralityof local area base station apparatuses 30, which each form a local areaC2 that is arranged in the wide area C1 and that is narrower than thewide area C1. Also, in the wide area C1 and each local area C2, manymobile terminal apparatuses 10 are arranged. The mobile terminalapparatuses 10 support the radio communication schemes for the wide areaand for the local areas, and are configured to be able to perform radiocommunication with the wide area base station apparatus 20 and the localarea base station apparatuses 30.

Communication between the mobile terminal apparatuses 10 and the widearea base station apparatus 20 is carried out using a wide area carrier(for example, a carrier having a narrow bandwidth in a low frequencyband). Communication between a mobile terminal apparatus 10 and a localarea base station apparatus 30 is carried out using a local area carrier(for example, a carrier having a wide bandwidth in a high frequencyband). Also, the wide area base station apparatus 20 and each local areabase station apparatus 30 are connected with each other by wireconnection or by wireless connection.

The wide area base station apparatus 20 and each local area base stationapparatus 30 are connected with a higher station apparatus, which is notillustrated, and are connected to a core network 40 via the higherstation apparatus. Note that the higher station apparatus may be, forexample, an access gateway apparatus, a radio network controller (RNC),a mobility management entity (MME) and so on, but is by no means limitedto these. Also, the local area base station apparatuses 30 may beconnected with the higher station apparatus via the wide area basestation apparatus 20.

Note that, although each mobile terminal apparatus 10 may be either anLTE terminal or an LTE-A terminal, in the following description, simplya mobile terminal apparatus will be described, unless specifiedotherwise. Also, although a mobile terminal apparatus will be describedto perform radio communication with the wide area base station apparatus20 and the local area base station apparatuses 30 for ease ofexplanation, more generally, user equipment (UE), including both mobileterminal apparatuses and fixed terminal apparatuses, may be used aswell. Also, the wide area base station apparatus 20 and the local areabase station apparatuses 30 may be referred to as wide area and localarea transmission points.

In the radio communication system, as radio access schemes, OFDMA(Orthogonal Frequency Division Multiple Access) is applied to thedownlink, and SC-FDMA (Single-Carrier Frequency-Division MultipleAccess) is applied to the uplink. OFDMA is a multi-carrier transmissionscheme to perform communication by dividing a frequency band into aplurality of narrow frequency bands (subcarriers) and mapping data toeach subcarrier. SC-FDMA is a single carrier transmission scheme toreduce interference between terminals by dividing, per terminal, thesystem band into bands formed with one or continuous resource blocks,and allowing a plurality of terminals to use mutually different bands.

Here, communication channels in the LTE system will be described.Downlink communication channels include a PDSCH (Physical DownlinkShared Channel), which is used by each mobile terminal apparatus 10 on ashared basis, and downlink L1/L2 control channels (PDCCH, PCFICH,PHICH). User data and higher control information are transmitted by thePDSCH. Scheduling information and so on for the PDSCH and the PUSCH aretransmitted by the PDCCH (Physical Downlink Control Channel). The numberof OFDM symbols to use for the PDCCH is transmitted by the PCFICH(Physical Control Format Indicator Channel). HARQ ACK and NACK for thePUSCH are transmitted by the PHICH (Physical Hybrid-ARQ IndicatorChannel). Also, in the LTE-A system, scheduling information for thePDSCH and the PUSCH and so on are transmitted by the EPDCCH (enhancedPhysical Downlink Control Channel).

Uplink communication channels include a PUSCH (Physical Uplink SharedChannel), which is used by each mobile terminal apparatus on a sharedbasis as an uplink data channel, and a PUCCH (Physical Uplink ControlChannel), which is an uplink control channel. User data and highercontrol information are transmitted by this PUSCH. Also, downlink radioquality information (CQI: Channel Quality Indicator), ACK/NACK and so onare transmitted by the PUCCH.

An overall configuration of a mobile terminal apparatus 10 will bedescribed with reference to FIG. 15. A mobile terminal apparatus 10 has,as processing sections of the transmitting sequence, a format selectionsection 101, an uplink signal generating section 102, an uplink signalmultiplexing section 103, baseband transmission signal processingsections 104 and 105, and RF transmitting circuits 106 and 107.

The format selection section 101 selects the transmission format for thewide area and the transmission format for the local areas. The uplinksignal generating section 102 generates uplink data signals andreference signals. In the event of the transmission format for the widearea, the uplink signal generating section 102 generates the uplink datasignal and reference signals for the wide area base station apparatus20. Also, in the event of the transmission format for the local areas,the uplink signal generating section 102 generates the uplink datasignal and reference signals for a local area base station apparatus 30.

The uplink signal multiplexing section 103 multiplexes the uplinktransmission data and the reference signals. An uplink signal for thewide area base station apparatus 20 is input in the basebandtransmission signal processing section 104, and subjected to digitalsignal processing. For example, in the event of an uplink signal of theOFDM scheme, the signal is converted from a frequency domain signal intoa time sequence signal through an inverse fast Fourier transform (IFFT),and has cyclic prefixes inserted therein. Then, the uplink signal passesthe RF transmitting circuit 106, and is transmitted from atransmitting/receiving antenna 110 for the wide area, via a duplexer 108that is provided between the transmitting sequence and the receivingsequence. In the transmitting/receiving sequences for the wide area,simultaneous transmission/reception is made possible by the duplexer108.

An uplink signal for a local area base station apparatus 30 is input inthe baseband transmission signal processing section 105, and subjectedto digital signal processing. For example, in the event of an uplinksignal of the OFDM scheme, the signal is converted from a frequencydomain signal into a time sequence signal through an inverse fastFourier transform (IFFT), and has cyclic prefixes inserted therein.Then, the uplink signal passes the RF transmitting circuit 107, and istransmitted from a transmitting/receiving antenna 111 for the wide area,via a change switch 109 that is provided between the transmittingsequence and the receiving sequence. In the transmitting/receivingsequences for the local areas, transmission and reception are switchedby a change switch 109.

Note that, although the present embodiment is configured to provide theduplexer 108 in the transmitting/receiving sequences for the wide areaand provide the change switch 109 in the transmitting/receivingsequences for the local areas, this configuration is by no meanslimiting. It is equally possible to provide the change switch 109 in thewide area transmitting/receiving sequences and provide the duplexer 108in the local area transmitting/receiving sequences. Also, uplink signalsfor the wide area and the local areas may be transmitted simultaneouslyfrom the transmitting/receiving antennas 110 and 111, or may betransmitted separately by switching between the transmitting/receivingantennas 110 and 111.

Also, the mobile terminal apparatus 10 has, as processing sections ofthe receiving sequence, RF receiving circuits 112 and 113, basebandreceived signal processing sections 114 and 115, a local area controlinformation receiving section 116, a discovery signal receiving section117, a discovery signal measurement section 118, and downlink signaldemodulation/decoding sections 119 and 120.

A downlink signal from the wide area base station apparatus 20 isreceived at the wide area transmitting/receiving antenna 110. Thisdownlink signal is input in the baseband received signal processingsection 114 via the duplexer 108 and the RF receiving circuit 112, andsubjected to digital signal processing. For example, in the event of adownlink signal of the OFDM scheme, the cyclic prefixes are removed, andthe signal is converted from a time sequence signal to a frequencydomain signal through a fast Fourier transform (FFT).

The local area control information receiving section 116 receives localarea control information from the downlink signal for the wide area.Here, as the local area control information, control information fordiscovery signal (DS) reception, control information for DACHtransmission, and control information for EPDCCH reception are received.The local area control information receiving section 116 outputs thecontrol information for discovery signal (DS) reception to the discoverysignal receiving section 117, outputs the control information for DACHtransmission to the discovery signal measurement section 118, andoutputs the control information for EPDCCH reception to the downlinksignal demodulation/decoding section 120. Note that the local areacontrol information is received from wide area base station apparatus 20by means of, for example, higher layer signaling such as RRC signaling,and broadcast information.

The downlink data signal for the wide area is input in the downlinksignal demodulation/decoding section 119, and decoded (descrambled) anddemodulated in the downlink signal demodulation/decoding section 119. Adownlink signal from the local area base station apparatus 30 isreceived in the transmitting/receiving antenna 111 for the local areas.This downlink signal is input in the baseband received signal processingsection 115 via the change switch 109 and the RF receiving circuit 113,and is subjected to digital signal processing. For example, in the eventof a downlink signal of the OFDM scheme, the cyclic prefixes areremoved, and the signal is converted from a time sequence signal to afrequency domain signal through a fast Fourier transform (FFT).

The discovery signal receiving section 117 receives the discovery signalfrom the local area base station apparatus 30 based on the controlinformation for discovery signal (DS) reception input from the wide areacontrol information receiving section 116. The control information fordiscovery signal reception includes, for example, radio resourceinformation and signal sequence information for receiving the discoverysignal from the local area base station apparatus 30. The radio resourceinformation includes, for example, the transmission interval, thefrequency position, and the code of the discovery signal.

The discovery signal measurement section 118 measures the received powerof the discovery signal received in the discovery signal receivingsection 117. As the received power of the discovery signal, for example,the SINR (Signal-to-Interference and Noise power Ratio) may be measured.

Also, the discovery signal measurement section 118 may calculate thepath loss of the discovery signal based on the received power of thediscovery signal. For example, the discovery signal measurement section118 calculates the path loss of the discovery signal based on thedifference between the transmission power value of the discovery signaland the received power value that is measured. In this case, thetransmission power of the discovery signal may be received from the widearea base station apparatus 20 as local area control information or maybe received from the local area base station apparatus 30. Thecalculated path loss is output to the uplink transmission power controlsection 122.

The received power of the discovery signal measured in the discoverysignal measurement section 118 and the path loss that is calculated istransmitted from the local area base station apparatus 30 using theDACH. Transmission by means of the DACH is carried out with the initialtransmission power determined in an uplink transmission power controlsection 122, which will be described later.

Transmission by means of the DACH is carried out based on the controlinformation for DACH transmission input from the local area controlinformation receiving section 116. The control information for DACHtransmission includes, for example, radio resource information and DM-RSsequence information for transmission to the local area base stationapparatus 30 by means of the DACH. The radio resource informationincludes, for example, the transmission interval, the frequencyposition, and the code of the DACH. The transmission time interval ofthe DACH may be set shorter than the transmission time interval for thediscovery signal so that the DACH is transmitted more frequently thanthe discovery signal, or may be set the same. In the DACH, user IDs maybe transmitted with the discovery signal measurement result.

Note that the received power or path loss of the discovery signal may betransmitted to the local area base station apparatus 30 using uplinkchannels (for example, the PUCCH and the PUSCH) apart from the DACH,using higher layer signaling, and so on. Also, both the received powerand the path loss of the discovery signal may be transmitted to thelocal area base station apparatus 30. Also, the received power or thepath loss of the discovery signal may be transmitted to the wide areabase station apparatus 20 using higher layer signaling. Also, both thereceived power and the path loss of the discovery signal may betransmitted to the wide area base station apparatus 20 or to the localarea base station apparatus 30.

The channel estimation section 121 performs channel estimation based onthe received power of the measurement reference signal (CSI-RS) from thelocal area base station apparatus 30. Note that, for the received powerof the CSI-RS, for example, the SINR (Signal-to-Interference and Noisepower Ratio) may be used.

Also, the channel estimation section 121 generates channel stateinformation (CSI), which represents the estimated channel state. ThisCSI includes a CQI (Channel Quality Indicator), a PMI (Precoding MatrixIndicator), an RI (Rank Indicator) and so on. The CSI is transmitted tothe local area base station apparatus 30 using the PUCCH or the PUSCH.Note that the CSI may be transmitted to the wide area base stationapparatus 20. Also, the CQI is calculated based on the SINR of theCSI-RS. This CQI may be corrected based on the transmission powerdifference between the CSI-RS and the EPDCCH or the PDSCH reported inadvance from the wide area base station apparatus 20 or the local areabase station apparatus 30. Alternatively, the CQI may have an expandedmeasurement range (expanded number of feedback bits) so that the SINR ofthe CSI-RS up to −20 dB can be measured, for example.

A downlink data signal for the local areas is input in a downlink signaldemodulation/decoding section 120, and decoded (descrambled) anddemodulated in the downlink signal demodulation/decoding section 120.Also, based on the control information for EPDCCH reception input fromthe wide area control information receiving section 116, the downlinksignal demodulation/decoding section 120 decodes (descrambles) anddemodulates the enhanced downlink control signal (EPDCCH) for the localarea. The control information for EPDCCH reception includes, forexample, radio resource information and DM-RS sequence information forreception from the local area base station apparatus 30 by means of theEPDCCH. The radio resource information includes, for example, thetransmission interval, the frequency position, and the code of theEPDCCH.

Also, downlink signals for the wide area and the local areas may bereceived simultaneously from the transmitting/receiving antennas 110 and111, or may be received separately by switching between thetransmitting/receiving antennas 110 and 111.

The uplink transmission power control section 122 controls thetransmission power of uplink signals for the local area base stationapparatus 30. To be more specific, the uplink transmission power controlsection 122 determines the initial transmission power of the DACH basedon the path loss of the discovery signal. For example, the uplinktransmission power control section 122 adds a predetermined offset tothe above path loss, at or below the maximum transmission power of themobile terminal apparatus 10, and determines the initial transmissionpower of the DACH. Also, the uplink transmission power control section122 determines the transmission power of the uplink data signal (PUSCH)based on the channel estimation result in the channel estimation section121. Note that the uplink transmission power control section 122 maycontrol the transmission power of the uplink data signal (PUSCH) basedon command information (for example, TPC commands) from the local areabase station 30.

An overall configuration of the wide area base station apparatus 20 willbe described with reference to FIG. 16. The wide area base stationapparatus 20 has, as processing sections of the transmitting sequence, alocal area control information generating section 201, a downlink signalgenerating section 202, a downlink signal multiplexing section 203, abaseband transmission signal processing section 204, and an RFtransmitting circuit 205.

The local area control information generating section 201 generates, aslocal area control information, control information for discovery signal(DS) transmission, control information for discovery signal (DS)reception, control information for DACH transmission, and controlinformation for EPDCCH reception. The local area control informationgenerating section 201 outputs the control information for discoverysignal transmission to a transmission path interface 211, and outputsthe control information for discovery signal reception, the controlinformation for DACH transmission, and the control information forEPDCCH reception to the downlink signal multiplexing section 203. Thecontrol information for discovery signal transmission is transmitted tothe local area base station apparatus 30 via the transmission pathinterface 211. Meanwhile, the control information for discovery signalreception, the control information for DACH transmission, and thecontrol information for EPDCCH reception are transmitted to the mobileterminal apparatus 10 via the downlink signal multiplexing section 203.

The downlink signal generating section 202 generates the downlink datasignal (PDSCH) and reference signals. The downlink signal multiplexingsection 203 multiplexes the local area control information, the downlinkdata signal (PDSCH) and the reference signals. A downlink signal for themobile terminal apparatus 10 is input in the baseband transmissionsignal processing section 204, and subjected to digital signalprocessing. For example, in the event of a downlink signal of the OFDMscheme, the signal is converted from a frequency domain signal to a timesequence signal through an inverse fast Fourier transform (IFFT), andhas cyclic prefixes inserted therein. Then, the downlink signal passesthe RF transmitting circuit 205, and is transmitted from thetransmitting/receiving antenna 207 via a duplexer 206 that is providedbetween the transmitting sequence and the receiving sequence.

Also, the wide area base station apparatus 20 has, as processingsections of the receiving sequence, an RF receiving circuit 208, abaseband received signal processing section 209, an uplink signaldemodulation/decoding section 210, a measurement information receivingsection 212, and a local area downlink transmission power determiningsection 213.

An uplink signal from the mobile terminal apparatus 10 is received inthe transmitting/receiving antenna 207, and is input in the basebandreceived signal processing section 209 via the duplexer 206 and the RFreceiving circuit 208. In the baseband received signal processingsection 209, the uplink signal is subjected to digital signalprocessing. For example, in the event of an uplink signal of the OFDMscheme, the cyclic prefixes are removed, and the signal is convertedfrom a time sequence signal to a frequency domain signal through a fastFourier transform (FFT). The uplink data signal is input in the uplinksignal demodulation/decoding section 210, and decoded (descrambled) anddemodulated in the uplink signal demodulation/decoding section 210.

The measurement information receiving section 212 receives themeasurement information transmitted from the mobile terminal apparatus10 through higher layer signaling. To be more specific, the measurementinformation received 212 receives the received power or path loss of thediscovery signal in the mobile terminal apparatus 10. The measurementinformation receiving section 212 may calculate the path loss based onthe difference between the transmission power value and the receivedpower value of the discovery signal from the local area base stationapparatus 30. The measurement information receiving section 212 outputsthe path loss of the discovery signal to the local area downlinktransmission power determining section 213.

Also, the measurement information receiving section 212 acquires channelstate information (CSI) that is estimated based on the received power ofthe measurement reference signal (CSI-RS) in the mobile terminalapparatus 10. The measurement information receiving section 212 outputsthe acquired CSI to the local area downlink transmission powerdetermining section 213.

The local area downlink transmission power determining section 213determines the transmission power of downlink signals from the localarea base station apparatus 30 to the mobile terminal apparatus 10. Tobe more specific, the local area downlink transmission power determiningsection 213 determines the initial transmission power of downlinksignals based on the path loss of the discovery signal. Also, the localarea downlink transmission power determining section 213 determines thetransmission power of downlink signals based on CSI and the path loss ofthe discovery signal. The local area downlink transmission powerdetermining section 213 transmits transmission power information, whichrepresents the transmission power that is determined, to the local areabase station apparatus 30, via the transmission path interface 211.

Note that the downlink signals, the transmission power of which is to bedetermined, may be the downlink data signal (PDSCH) from the local areabase station apparatus 30, enhanced downlink control signal (EPDCCH) andso on. Also, when the transmission power of downlink signals isdetermined in the local area base station apparatus 30, the local areadownlink transmission power determining section 213 may be omitted.

An overall configuration of the local area base station apparatus 30will be described with reference to FIG. 17. Assume that the local areabase station apparatus 30 is arranged very close to the mobile terminalapparatus 10. The local area base station apparatus 30 has, asprocessing sections of the transmitting sequence, a local area controlinformation receiving section 301, a discovery signal generating section302, a downlink signal generating section 303, a reference signalgenerating section 304, a downlink signal multiplexing section 305, abaseband transmission signal processing section 306, and an RFtransmitting circuit 307.

The local area control information receiving section 301 receives localarea control information from the wide area base station apparatus 20via the transmission path interface 314. Here, for the local areacontrol information, control information for discovery signaltransmission is received. The local area control information receivingsection 301 outputs the control information for discovery signaltransmission to the discovery signal generating section 302.

The discovery signal generating section 302 generates the discoverysignal (detection signal), which is used to detect a local area basestation apparatus 30 in the mobile terminal apparatus 10, based on thecontrol information for discovery signal (DS) transmission input fromthe local area control information receiving section 301. The controlinformation for discovery signal reception includes, for example, radioresource information and signal sequence information for receiving thediscovery signal from the local area base station apparatus 30. Theradio resource information includes, for example, the transmissioninterval, the frequency position, and the code of the discovery signal.Note that the transmission power of the discovery signal is set to afixed value so as to have wider coverage than downlink signals, whichwill be described later.

The downlink signal generating section 303 generates downlink signalsfor the mobile terminal apparatus 10. To be more specific, the downlinksignal generating section 303 generates the downlink data signal (PDSCH)and the enhanced downlink control signal (EPDCCH) to befrequency-division-multiplexed upon the downlink data signal (PDSCH).The transmission power of the downlink data signal and the downlinkcontrol signal is controlled adaptively by a downlink transmission powercontrol section 315, which will be described later.

The reference signal generating section 304 outputs reference signalssuch as the measurement reference signal (CSI-RS) and the demodulationreference signal (DM-RS), and outputs these to the downlink signalmultiplexing section 305. The transmission power of the measurementreference signal (CSI-RS) is used in channel state estimation in themobile terminal apparatus 10, and therefore is set to a fixed value.

Also, the measurement reference signal (CSI-RS) is multiplexed withdownlink signals in the downlink signal multiplexing section 305, and,in the baseband transmission signal processing section 306, arranged ina predetermined radio resource region using a predetermined arrangementpattern. For example, the CSI-RS may be arranged at shorter subcarrierintervals than the CSI-RS that is transmitted in the wide area, inspecific OFDM symbols in a subframe, as shown in FIG. 12A. Also, asshown in FIG. 12B, the CSI-RS may be arranged in all the subcarriersconstituting the resource block width in specific OFDM symbols in asubframes. Also, as shown in FIG. 12C, the CSI-RS may be arranged inpart of the subcarriers constituting the resource block width inspecific OFDM symbols in a subframe, and muting resources may bearranged in the rest of the subcarriers. Also, when the wide areacarrier and the local area carrier belong to different componentcarriers by means of carrier aggregation, the CSI-RS of the local areamay be arranged in a subframe that is different from the subframe wherethe CSI-RSs that is transmitted in the wide area is arranged, as shownin FIG. 13.

The downlink signal multiplexing section 305 multiplexes the downlinksignals generated in the downlink signal generating section 303 and thereference signals generated in the reference signal generating section304. The downlink signals multiplexed with the reference signals areinput in the baseband transmission signal processing section 306, andsubject to digital signal processing. For example, in the event of adownlink signal of the OFDM scheme, the signal is converted from afrequency domain signal to a time sequence signal through an inversefast Fourier transform (IFFT), and has cyclic prefixes inserted therein.Then, the downlink signal passes the RF transmitting circuit 307, and istransmitted from a transmitting/receiving antenna 309 via the changeswitch 308 that is provided between the transmitting sequence and thereceiving sequence. Note that a duplexer may be provided instead of thechange switch 308.

The local area base station apparatus 30 has, as processing sections ofthe receiving sequence, an RF receiving circuit 310, a baseband receivedsignal processing section 311, an uplink signal demodulation/decodingsection 312, and a measurement information receiving section 313.

An uplink signal from the mobile terminal apparatus 10 is received inthe transmitting/receiving antenna 309 for the local areas, and input inthe baseband received signal processing section 311 via the changeswitch 308 and the RF receiving circuit 310. In the baseband receivedsignal processing section 311, the uplink signal is subjected to digitalsignal processing. For example, in the event of an uplink signal of theOFDM scheme, the cyclic prefixes are removed, and the signal isconverted from a time sequence signal to a frequency domain signalthrough a fast Fourier transform (FFT). The uplink data signal is inputin the uplink signal demodulation/decoding section 312, and decoded(descrambled) and demodulated in the uplink signal demodulation/decodingsection 312.

The measurement information receiving section 313 receives measurementinformation of the discovery signal. To be more specific, themeasurement information receiving section 313 receives the receivedpower or the path loss of the discovery signal transmitted from themobile terminal apparatus 10. The measurement information receivingsection 313 may calculate the path loss based on the difference betweenthe transmission power value and the received power value of thediscovery signal. The measurement information receiving section 313outputs the path loss of the discovery signal to the downlinktransmission power control section 315.

Also, the measurement information receiving section 313 receives thechannel state information (CSI) estimated in the mobile terminalapparatus 10 based on the received power of the measurement referencesignal (CSI-RS). The measurement information receiving section 313outputs the acquired CSI to the downlink transmission power controlsection 315.

The downlink transmission power control section 315 controls thetransmission power of downlink signals for the mobile terminal apparatus10. To be more specific, the initial transmission power of downlinksignals is determined based on the path loss of the discovery signalinput from the measurement information receiving section 313. Also, thedownlink transmission power control section 315 determines thetransmission power of downlink signals based on the CSI and the pathloss of the discovery signal. The downlink transmission power controlsection 315 controls the downlink signal generating section 303 so thatdownlink signals are transmitted with the determined transmission power.Note that the downlink signals, the transmission power of which is to bedetermined, may be the downlink data signal (PDSCH), the enhanceddownlink control signal (EPDCCH) and so on. Note that the downlinktransmission power control section 315 may control the transmissionpower of the downlink signals based on command information (for example,TPC commands) from the mobile terminal apparatus 10 in closed loopcontrol.

Also, the downlink transmission power control section 315 may determinethe initial transmission power and the transmission power of downlinksignals for the mobile terminal apparatus 10 based on transmission powerinformation received from the wide area base station apparatus 20 viathe transmission path interface 314.

As described above, with the radio communication system 1 according tothe present embodiment, the transmission power of downlink signals iscontrolled adaptively, even in the downlink of a local area, based onthe path loss of the discovery signal. In this way, by controlling thetransmission power of a local area carrier not only in the uplink of thelocal area, but also in the downlink the local area as well, it becomespossible to make the uplink and downlink coverages in the local areanearly symmetrical. Consequently, it is possible to provide highlyefficient local area radio access that is specifically designed forlocal areas.

The present invention is by no means limited to the above embodiment andcan be implemented in various modifications. For example, withoutdeparting from the scope of the present invention, it is possible toadequately change the number of carriers, the bandwidth of carriers, thesignaling method, the type of the additional carrier type, the number ofprocessing sections, the order of processing steps in the abovedescription, and implement the present invention. Also, FIG. 4 is simplyan example, and it is equally possible to use a carrier of a highfrequency band in the wide area as well. Besides, the present inventioncan be implemented with various changes, without departing from thescope of the present invention.

The invention claimed is:
 1. A mobile terminal apparatus thatcommunicates with a first radio base station forming a first cell and asecond radio base station forming a second cell, the mobile terminalapparatus comprising: a receiver that receives, from the first radiobase station that is different from the second radio base station,information about a transmission power difference in decibels, dB,between transmission power of a downlink signal and transmission powerof a CSI-RS (Channel State Information Reference Signal) in the secondcell formed by the second base station; a processor that uses theinformation as a basis to generate a CQI (Channel Quality Indicator)used for transmission of the downlink signal in the second cell; and atransmitter that transmits the CQI by using a PUCCH (Physical UplinkControl Channel) in the second cell.
 2. The mobile terminal apparatusaccording to claim 1, wherein the receiver receives the information fromthe first radio base station by Radio Resource Control (RRC) signaling.3. A second radio base station forming a second cell, the second radiobase station comprising: a receiver that receives CQI (Channel QualityIndicator) that is generated based on a CSI-RS (Channel StateInformation Reference Signal) in the second cell formed by the secondbase station; and a transmitter that transmits a downlink signal in thesecond cell based on the CQI, wherein the CQI is generated based oninformation about a transmission power difference in decibels, dB,between transmission power of the downlink signal and transmission powerof the CSI-RS and the information is transmitted from a first radio basestation forming a first cell, and the first radio base station isdifferent from the second base station.
 4. The second radio base stationaccording to claim 3, wherein the information is transmitted from thefirst radio base station by Radio Resource Control (RRC) signaling.
 5. Aradio communication method for a mobile terminal apparatus thatcommunicates with a first radio base station forming a first cell and asecond radio base station forming a second cell, the radio communicationmethod comprising: receiving, from the first radio base station that isdifferent from the second radio base station, information about atransmission power difference in decibels, dB, between transmissionpower of a downlink signal and transmission power of a CSI-RS (ChannelState Information Reference Signal) in the second cell formed by thesecond base station; using the information as a basis to generate a CQI(Channel Quality Indicator) used for transmission of the downlink signalin the second cell; and transmitting the CQI by using a PUCCH (PhysicalUplink Control Channel) in the second cell.